This application claims the benefit of French Patent Application, Serial Number 00 03771, filed Mar. 24, 2000 entitled Process And Device For Growing Single Crystals, Especially Of Caf2, by Patrick Herve.
The present invention relates to a process for growing single crystals and to an associated device suitable for carrying out said process. Said process and device are very particularly suitable for growing single crystals of calcium fluoride (CaF2).
Ultra-high-performance optical systems are required to increase the level of integration of electronic components on a semiconductor wafer insofar as a radiating light of very low UV wavelength (below 248 nm) is necessary to improve the resolution.
To obtain such optical systems, the most widespread technique to date uses fused silica. Another technique already being exploited, uses single crystals of calcium fluoride (CaF2).
Such single crystals or single crystals of the same type (single crystals of alkaline earth metal fluoride, in general, or even single crystals of silica) are obtained by the so-called Stockbarger or Bridgman method. In said method the crystals are generated in a furnace, inside which a crucible containing the molten material is moved from top to bottom, along a vertical axis, from a hot zone into a cold zone. The temperature of said hot zone is maintained above the melting point of the material in question (in the case of CaF2 it is above 1525xc2x0 C.). The crucible advances at a speed of about 0.3 to 5 mm/h.
As it passes from the hot zone to the cold zone, the material goes through a zone of high thermal gradient. Crystallization takes place inside the crucible when the material reaches the zone in which the temperature is below its melting point. The fixed crystallization front propagates inside the crucible, within the material, from bottom to top, insofar as said crucible is caused to move downwards.
To prevent any oxidation of the material and the components of the furnace, said furnace is generally maintained under vacuum. The crucible is made of a material resistant to chemical attack and is generally a graphite crucible.
In fact, said method is mainly carried out according to two variants.
According to the first variant, which is carried out batchwise, a stack of crucibles (each crucible in said stack containing the material in question) is loaded cold into the upper zone of a tower furnace, which is to become the hot zone of said furnace. The furnace, loaded in its upper zone, is heated. In a first stage, the stack of crucibles is kept hot, at a temperature above the melting point of the material in question, in said upper zone, or hot zone, of said furnace. In a second stage, said stack is lowered into and held in the lower zone, or cold zone, of said furnace, which is maintained at a temperature below that of the hot zone and below the melting point of said material in question.
The main disadvantages of this operational variant are its low productivity and mediocre yield. The productivity is hampered by dead times before and after the growth of the crystals (dead times for loading the furnace, evacuating said furnace, heating the two zones of said furnace, cooling said two zones and unloading said furnace). The yield is affected by the fact that each crucible in the stack is not exposed to the same thermal conditions over a complete cycle of said stack. In particular, the crucible at the bottom of said stack is brought into contact with the cold zone more rapidly and remains in said cold zone for longer. In addition, such discontinuous thermal conditions (hot zone/cold zone) inside one and the same furnace are difficult to control.
According to the second variant, which is described in Russian author""s certificate no. 2161891 filed on Aug. 8, 1975, the aim is to carry out a continuous heat treatment. A stack of crucibles occupies the whole height of a tower furnace, which has two superposed thermal treatment chambers. When the stack undergoes a translational movement from top to bottom, each crucible in said stack passes successively through the upper chamber and then the lower chamber. For the emptying of each crucible at the bottom of the stack (the crucible which has therefore passed successively through said two thermal treatment chambers), the vertical translational movement of said stack is stopped (the crucible surmounting said crucible at the bottom of the stack being clamped between jaws for supporting and stabilizing said stack). This involves a degree of discontinuity in the thermal treatment and in any case entails decelerations and accelerations of the movement of the stack, and jerks, which are responsible for vibrations within the treated material. This is highly detrimental to optimized growth of the desired single crystals.
In such a context, the inventors designed and developed an optimized process for growing single crystals. Said process is also based on the so-called Stockbarger or Bridgman method explained above. It is optimized in that it is a truly continuous thermal treatment process which ensures that each crucible in the stack moving translationally through the furnace has the same history (very particularly as regards the thermal conditions), without jerks and with no vibration.
The prior art mentioned above and the invention mentioned below are explained only in terms of the mechanical aspect of the process (and its associated device) in question; the chemical aspect, and especially the advantageous presence of a fluorinating agent, has been generally described in the literature and is not repeated here.
The process and device of the invention are referred to in the present text as a process and device for growing single crystals. This description cannot imply a limitation. They can be described more generally as a process and device for growing crystals (polycrystals and monocrystals) insofar as they are obviously suitable for generating polycrystals. Their use for generating such polycrystals only is not excluded from the framework of the present invention (although it hardly seems pragmatic insofar as polycrystals can be obtained much more simply). The process and device of the invention can be described more precisely as a process and device for growing crystals which have been optimized to give a valuable yield of single crystals.
The process of the invention is therefore a process for growing single crystals (of the CaF2, BaF2, Magnesium Fluoride, optical fluoride crystals), carried out in the absence of impurities (under vacuum, in general, and/or in a controlled atmosphere) in a tower furnace inside which two superposed thermal treatment chambers are arranged, namely a first chamber, or melting chamber, and a second chamber, or annealing chamber, a sizeable thermal gradient being created between said first and second chambers.
Said process of the invention comprises:
the support and the translational movement along a vertical axis, inside said furnace, of a stack of crucibles containing the starting material (said starting material, inside each crucible, generally being introduced in the form of a powder or a previously melted disk), the height of said stack of crucibles, in operation, being greater than the sum of the heights of said superposed first and second chambers and each stack being supported and moved translationally in a direction such that each of the crucibles constituting said stack passes successively through said first chamber and then said second chamber under the action of means acting on at least the crucible at the bottom of said stack and arranged in a third chamber, or translation chamber, positioned underneath said pair of first and second chambers; and
the loading of a new crucible, upstream of said first chamber, at one end of said stack, and the unloading of the crucible which has successively passed through said first and second chambers, at the other end of said stack, the loading and unloading operations being performed at the same frequency so that the height of said stack, in operation, is kept substantially constant.
As such, the process of the invention is of the same type as that described in the Russian author""s certificate mentioned above. However, it can be pointed out straightaway that, in the process of the invention, the stack of crucibles can move either upwards or downwards according to the positioning of the superposed melting and annealing chambers.
In a first variant, the stack of crucibles is moved translationally from top to bottom, in which case the first (melting) chamber surmounts the second (annealing) chamber, which itself surmounts the translation chamber.
In a second variant, the stack of crucibles is moved translationally from bottom to top, in which case the first (melting) chamber surmounts the translation chamber and is positioned underneath the second (annealing) chamber.
Said first variant is a priori preferred.
The thermal conditions assured in each of the two melting and annealing chambers, and the resulting thermal gradient, are those which are necessary to obtain the expected effect, namely the formation and growth of single crystals inside the crucibles.
Characteristically, within the framework of the process of the invention as specified above, the operations of loading (of the crucibles filled with starting material) and unloading (of said filled crucibles after they have successively passed through the two superposed thermal treatment chambers) are performed without stopping the translational movement of the stack of crucibles inside the furnace. Said translational movement is executed in a perfectly continuous manner. According to the invention, said stack is driven continuously and smoothly. The heat treatment applied in this way is perfectly continuous, perfectly identical for all the crucibles and completely smooth.
The process of the invention as described above is generally started with a first stack of crucibles called a priming stack. This can be a stack of empty crucibles or straightaway a stack of filled crucibles, in which case the contents of said crucibles in said priming stack are to be discarded.
Within the framework of the implementation of the process of the invention, the stack of filled crucibles can be caused to move only with the continuous translational movement which successively conveys each crucible through the melting chamber and then through the annealing chamber. It is advantageously caused to move both with said continuous translational movement and with a rotational movement about itself. This optimizes the homogeneity of the heat treatment in the bulk of the material inside each crucible in said stack.
For the support and movement of said stackxe2x80x94simple translational movement or combined translational and rotational movement, in either case perfectly continuousxe2x80x94appropriate means, arranged in the translation chamber, act on at least the crucible at the bottom of the stack, as indicated above. Obviously, for said stack to be stable and driven perfectly, said means advantageously act on at least the two crucibles (2, 3 or even 4 crucibles) at the bottom of said stack.
Within the framework of the preferred variant of the implementation of the process of the invention, said means act on the side wall of at least the crucible at the bottom of said stack, advantageously on the side walls of at least the two crucibles (2, 3 or even 4 crucibles) at the bottom of said stack. This type of lateral action is particularly appropriate for ensuring that the stack is driven continuously and without interruption.
Reference has been made to the side wall, in the singular, of the crucibles inasmuch as the crucibles are generally cylindrical (and therefore have only one side wall). This singular in no way implies a limitation. Were parallelepipedal crucibles to be usedxe2x80x94a priori an unlikely possibilityxe2x80x94the means for supporting and moving the stack obviously act advantageously on at least two of the side walls (advantageously opposite walls) of at least the crucible at the bottom of the stack.
There are different possible ways of loading a new crucible and unloading a crucible which has successively passed through the two superposed thermal treatment chambers. In particular, said operations can be based on the technique described in the Russian author""s certificate identified above. Advantageously, within the framework of the implementation of the process of the invention, the loading operation and/or (advantageously and) the unloading operation are (is) performed along the axis of the stack of crucibles. Said axis of the stack of crucibles preferably corresponds to the axis of the furnace inside which said stack is being conveyed.
As already indicated, the process of the invention as described in general terms above is perfectly suitable for growing (single) crystals of CaF2.
Preferred variants of the implementation of said process are described later in the present text with reference to the attached Figures, without implying any limitation.
According to its second subject, the present invention relates to a device suitable for carrying out said process, i.e. a device suitable for the continuous growing of single crystals, said device comprising the following, arranged in an enclosure with a vertical axis:
three superposed chambers, namely a first thermal treatment chamber, or melting chamber, a second thermal treatment chamber, or annealing chamber, and a third chamber, or translation chamber, positioned underneath said pair of first and second chambers;
heating means, associated with each of said two thermal treatment chambers, for maintaining appropriate thermal treatment temperatures inside said chambers with a sizeable thermal gradient between said two chambers;
means for assuring the support and the translational movement along a vertical axis, through said three superposed chambers, of a stack of crucibles, said means being arranged in said third chamber, acting on at least the crucible at the bottom of said stack and assuring said translational movement in a direction such that each of the crucibles in said stack passes successively through said first chamber and then said second chamber;
means for either:
bringing said first chamber into communication with a crucible loading zone, at the top of said stack, and bringing said third chamber into communication with a crucible unloading zone, at the bottom of said stack, when said stack of crucibles is moved translationally from top to bottom; or
bringing said third chamber into communication with a crucible loading zone, at the bottom of said stack, and bringing said second chamber into communication with a crucible unloading zone, at the top of said stack, when said stack of crucibles is moved translationally from bottom to top;
means for assuring the loading and unloading operations; and
means for keeping said three superposed chambers and said loading and unloading zones free of impurities;
said means for assuring the support and the translational movement of said stack, said communication means and said means for assuring the loading and unloading operations cooperating so that said loading and unloading operations are assured without stopping the translational movement of said stack.
Said means cooperate in order to assure a continuous translational movement of said stack.
Said enclosure (the crystallization furnace) therefore comprises the following, from top to bottom:
in a first variant:
+the loading zone,
+the melting chamber,
+the annealing chamber,
+the translation chamber,
+the unloading zone,
in which first variant the means for assuring the support and the translational movement of the stack of crucibles assure said translational movement from top to bottom;
in a second variant:
+the unloading zone,
+the annealing chamber,
+the melting chamber,
+the translation chamber,
+the loading zone,
in which second variant the means for assuring the support and the translational movement of the stack of crucibles assure said translational movement from bottom to top.
The device of the invention is advantageously arranged according to the first variant above.
Within the framework of one or other of said variants, said three superposed melting, annealing and translation chambers advantageously have the same axis, which preferably coincides with the axis of the stack of crucibles, and said loading zone or/and (advantageously and) said unloading zone is (are) arranged along this same axis.
To obtain the expected resultxe2x80x94the loading and unloading of crucibles without stopping the continuous translational movement of the stack of cruciblesxe2x80x94by cooperation between the appropriate means indicated above, there are advantageously means for assuring the support and the translational movement of said stack which act on the side wall of at least the crucible at the bottom of said stack, and which advantageously act on the side walls of at least the two crucibles at the bottom of said stack.
Said means which assure the support and the continuous translational movement of said stack constitute the key means of the device of the invention. They can exist in different forms. In particular, they can be suitable for assuring only a simple translational movement of said stack; they can also be suitable for assuring both a translational movement and a rotational movement of said stack about itself.
Three types of such means are specified below by way of illustration. The first two types are suitable for a simple translational movement and the third is suitable for translational movement combined with rotational movement.
Said means for assuring the support and the continuous translational movement of the stack of crucibles inside the enclosure with a vertical axis (tower furnace) can comprise the following in particular:
at least one set of at least two rollers with horizontal axes, acting on the side wall of the crucible at the bottom of the stack, and advantageously, in addition to said set of rollers, at least one other set of rollers of the same type, acting on at least one other crucible at the bottom of the stack (advantageously acting on the crucible directly above said crucible at the bottom of said stack). The set(s) of such rollers advantageously comprises (comprise) four rollers 90xc2x0 apart. It is thus recommended to use two superposed sets (the one acting on the crucible at the bottom of the stack and the other acting on the crucible just above) of four rollers each, made of a suitable material (1st type).
at least two sets of at least two jaws each, each of said sets of jaws being suitable for assuring the support of said stack by clamping of the crucible at the bottom of said stack and being capable of being retracted from said stack and caused to move translationally along a vertical axis, in alternation with the other set of jaws, in order to assure the continuous translational movement of said stack (2nd type).
Said means for assuring the support and the continuous translational/rotational movement of the stack of crucibles inside the enclosure with a vertical axis (tower furnace) can comprise the following in particular:
at least one set of at least two rollers, whose axis is slightly inclined relative to the vertical, acting on the side wall of the crucible at the bottom of said stack, and advantageously, in addition to said set of rollers, at least one other set of rollers of the same type, acting on at least one other crucible at the bottom of said stack (advantageously acting on the crucible directly above said crucible at the bottom of said stack). The set(s) of such rollers advantageously comprises (comprise) three rollers 120xc2x0 apart, made of a suitable material (3rd type). To drive the stack of crucibles with a rotational movement during its translational movement, each of the rollers in such a set is offset by an angle xcex1 relative to said stack. Each of said rollers actually rotates about an axis which is therefore not parallel to the longitudinal vertical axis of the device, nor does it intersect with said axis.
Preferred variants of the device of the invention are described in greater detail below with reference to the attached Figures.